The present invention relates to a displacent generating device and, more particularly, to a device for generating small, periodic displacements using an actuator comprising a piezoelectric element so as to successively displace the position of an object.
In a "reverse piezoelectric effect" known to those skilled in the art, a piezoelectric body is deformed in accordance with supplied electrical energy such as a voltage. When an AC voltage signal, which has a periodically changing voltage level, is applied, the extent of deformation of the piezoelectric body (i.e., the amount of displacement) also changes periodically. Thus, the piezoelectric body generates repetitive mechanical displacement (i.e., vibration). By using this principle, a piezoelectric element is used as an actuator in order to displace or vibrate a small object. A piezoelectric actuator has many advantages: it is compact, light weight, has a high response speed and low power consumption, generates no heat during vibration, has no adverse effects on peripheral circuits, and so on. Therefore, when the position of an object must be precisely set on the order of microns or submicrons, a piezoelectric actuator is widely adopted. For example, when a small chip (e.g., a magnetic head of a VTR) incorporated in high-precision electronic equipment is to be position controlled accurately or vibrated in a desired mode, a piezoelectric actuator is preferably used. Since a circuit for driving the piezoelectric actuator is relatively simple, it can be easily insta1led in a small space in highly integrated electronic equipment.
However, a piezoelectric actuator having the excellent characteristics described above suffers from a certain problem: it exhibits hysteresis characteristics in a displacement generation mode. More specifically, amounts of displacement produced at the same voltage level differ when a voltage applied to the piezoelectric actuator increases or decreases. In this case, hysteresis H (%) is expressed by: EQU H=(DH/D).times.100
where
DH: a difference between a residual amount of displacement generated in the piezoelectric actuator when an application voltage increases to zero, and that when it decreases to zero; and PA1 D: a maximum magnitude of the amount of displacement.
The fact that the piezoelectric actuator has the hysteresis characteristics means that amount of displacement of the piezoelectric actuator changes nonlinearly with respect to an application voltage. In other words, when a linearly changing drive voltage (e.g., a triangular wave voltage) is applied to the piezoelectric actuator, its actual amount of displacement changes nonlinearly. Therefore, the application voltage cannot have proportional, one-to-one correspondence with the amount of displacement of the piezoelectric actuator. As a result, an object mounted on the piezoelectric actuator cannot be precisely aligned. If an object is vibrated between two points using a piezoelectric actuator, a moving speed of the object between the two points changes because of the hysteresis characteristics, and a vibration pattern with ideal linear characteristics cannot be obtained.
In order to compensate for the hysteresis characteristics of a piezoelectric actuator as above, a level of the application voltage itself must be precisely controlled to compensate for an increase or decrease in the amount of displacement of the piezoelectric actuator, with respect to a constant application voltage. For example, in a conventional method, an increase or a decrease in the amount of displacement caused by the hysteresis characteristics of the piezoelectric actuator is detected in real time by a sensor, and the drive voltage level is precisely controlled in a feedback manner to compensate for the change in amount of displacement each time it is detected. However, since such closed loop voltage control for compensation of the hysteresis characteristics requires a feedback circuit including a sensor, its response time is not greatly improved. In order to improve this response time, the circuit configuration must be complicated, and it cannot be insta1led in highly integrated electronic equipment, resulting in a limited range of application for such a piezoelectric actuator.